DiMarzios do glue, I opened up two older Super Distortions, a Super 3 and an older PAF (not PAF Pro) from the top of my head and they all had their magnets glued in. It's a false statement that DiMarzio doesn't glue when, at least at one point in time, they did. This isn't only my experience, either.
That said, as I previously stated, that in itself isn't an issue really.
As far as I'm aware, both of the companies use the same manufacturer for bobbins, don't have a clue on manufacturer of wire, though I am sure different models use different ones (like Duncan Custom using "cheap" poly sourced from Japan, however as you can tell there's nothing cheap about it's sound), with the same going for magnets and baseplates. It's the kind of people that think they know something, when in reality they have little clue, that presume DiMarzios use inferior quality components.
You're looking at it the wrong way, there's (much?) more Duncan guys then DiMarzio guys, I'm affraid, so naturally they're modded more. Then there's the whole DiMarzio thing, you know, people buy DiMarzio for a specific sound - it's as if DiMarzio is made with an complete bluprint, you either like it for the specific guitar and music, or you simply get another DiMarzio. Duncans, on the other hand, tend to be more similar, and as such, it's naturally that people want to make things sound better, or at the very least, different, hence modding.
The furthest I usually go with mods, is quasy-air mode (plastic spacers), change of magnet type and sometimes switch from adjustable polepieces to hex if I want more clarity...nothing too extreme.